1. Field of the Invention
The present invention relates to an exposure control apparatus of a photo-printer, and more particularly to an exposure control apparatus of a photo-printer, which detects three color image data from an original frame to be printed, to determine print-exposure amounts of three colors, and displays a positive video image simulating a consequent photo-print that will be made from the original frame.
2. Background Arts
It is known in the field of photo-printers to extract image characteristic amounts of three primary colors from an original frame of a photo film, e.g. a color negative film, and calculate three color exposure amounts in accordance with the image characteristic amounts. Based on the three color exposure amounts, the quantity and quality of printing light are controlled to make a photo-print at proper color balance and density. It is also known in the art to extract a main subject from the original frame. For example, JPA 63-278047 discloses that the type of the main subject is manually entered to locate a point of the main subject of the original frame on a video monitor which displays a simulation image of a photo-print made from the original frame, that a main subject area of a given size is determined around the located point, that image characteristic amounts are calculated from photometric values of those pixels which constitute the main subject area, and that three color exposure amounts are calculated from the image characteristic amounts of the main subject area by use of an operation formula that is predetermined for each type of main subject.
Meanwhile, to detect color image data useful for simulating a consequent photo-print on a video monitor, a high-density image area sensor having hundreds of thousands of pixels is necessary. On the other hand, to detect photometric values useful for the exposure control in the photo-printer, spectral characteristics of a photometric system is required to be as equal to those of the used color photographic paper as possible. Therefore, the photometric system for exposure control needs to use an optical filter whose spectral characteristics are defined by inorganic deposit layers.
In view of the above two requirements, a three chip image sensing device has conventionally been used for picking up color image data from the original frame. The three chip image sensing device is provided with a three color separation prism having a deposit color separation system, i.e. inorganic three color filters. The color image data detected through the image sensing device is used for simulating the photo-print, and is also processed to obtain data necessary for controlling exposure. The conventional three chip image sensing device using the three color separation prism is complicated in construction, and expensive. Also, it is difficult to produce a color separation filter consisting of three color filters arranged in a mosaic pattern from inorganic deposit layers when the color image area sensor should have a large number of pixels, i.e. hundreds thousand of pixels. Moreover, pixel thinning and averaging the color image data in the unit of hundreds of thousands of pixels take a certain operation time, thereby elongating the image processing time per frame.